Stereoselective construction of fluorinated indanone derivatives via a triple cascade Lewis acid-catalyzed reaction

Site-selective synthesis of indanyl-substituted indole derivatives via 1,3-dithiane induced Nazarov cyclization

We report an efficient site-selective synthetic method to C2 and C3 indanyl-substituted indole derivatives via 1,3-dithianyl induced Nazarov-type cyclization. In particular, C2-selective indanyl-substituted indoles were directly obtained by a BF3·Et2O-promoted sequence of intramolecular C3-C2 migration and Nazarov-type cyclization process.

Divergent Reactivity of Triaryldivinyl Ketones: Competing 4π and Putative 6π Electrocyclization Pathways

This work describes an acid-promoted cyclization of triaryldivinyl ketones containing a thiophene moiety in the α-position. Two cyclization pathways are accessible: one a 4π-Nazarov cyclization and the other we propose to proceed through a 6π electrocyclic mechanism. The relative proportion of products from these divergent pathways is affected by reaction conditions and steric bulk in the substrate. We present experimental and computational evidence that when using HCl in dioxane, the 4π-conrotatory electrocyclization is more favorable, whereas GaCl₃ in methylene chloride shifts the chemoselectivity toward a putative 6π-disrotatory electrocyclization. DFT calculations suggest that a complex interplay between kinetic and thermodynamic factors is implicated in the chemodivergent behavior.

Cascade Cyclization of Azadienes with Difluoroenoxysilanes: A One-Pot Formal [4 + 2] Approach to Fluorinated Polyfused Heterocycles

A TfOH-promoted synthesis of fluorinated polyfused heterocycles via the cascade cyclization of azadienes and difluoroenoxysilanes has been developed, leading to the facile construction of fluorinated benzofuro[3,2-b]pyridines, 5H-indeno[1,2-b]pyridines, and 5,6-dihydrobenzo[h]quinolines. This one-pot formal [4 + 2] approach involves 1,4-difluoroalkylation, desulfonylation, cyclization, and dehydrated and dehydrofluorinated aromatization and represents the first application of difluoroenoxysilane in cascade transformations. Furthermore, this methodology is highlighted by the synthesis of three fluoro analogues of bioactive molecules with potent topoisomerase inhibitory activities.

Nickel/Copper Dual Catalysis for Sequential Nazarov Cyclization/Decarboxylative Aldol Reaction

A nickel/copper cocatalyzed sequential Nazarov cyclization/decarboxylative aldol reaction is reported. Various β-hydroxycyclopentenones containing three contiguous stereocenters were prepared in good yields with high diastereoselectivities. A gram-scale reaction and further derivatizations were easily achieved to highlight the synthetic utility of this transformation. Preliminary attempts for sequential asymmetric transformation were also conducted.

Interrupting the Nazarov Cyclization with Bromine

The generation of dibrominated cyclopentenones via an interrupted Nazarov cyclization is reported. The installation of two bromine atoms occurs at the α and α' positions of the cyclopentenyl scaffold via successive nucleophilic and electrophilic bromination of the 2-oxidocyclopentenyl cation and its resulting enolate. Notably, the reaction proceeds with good diastereoselectivity, favoring the symmetrical product.

Recent Progress on Nazarov Cyclizations: The Use of Iron Salts as Catalysts in Ionic Liquid Solvent Systems

Nazarov cyclization is an important and versatile method for the synthesis of five-membered carbocycles, and extensive studies have been conducted to optimize the reaction. Among recent studies, several trends are recognized. One is the combination of different reactions with Nazarov cyclization in a one-pot reaction system which enables the preparation of unique cyclization products. The second is the use of a transition-metal catalyst, though Lewis or Brønsted acids have generally been used for the reaction. The third is the realization of the asymmetric Nazarov cyclization. The fourth is the base-catalyzed Nazarov cyclization. Furthermore, several useful protocols for realizing Nazarov cyclization have also been developed. The recent progress on Nazarov cyclizations is summarized in Section 2. Section 3 is our chronicle in this field. We focused on the use of iron as the catalyst in Nazarov cyclizations and ionic liquids as solvents: Nazarov cyclization of thiophene derivatives using FeCl3 as the catalyst was accomplished and we succeeded in demonstrating the first example of an iron-catalyzed asymmetric Nazarov reaction. We next established Nazarov cyclization of pyrrole or indole derivatives using Fe(ClO4 )3 ·Al2 O3 as the catalyst with high trans selectivities in excellent yields. Since the cyclized product was reacted with a vinyl ketone in the presence of the same iron salt, the system allowed realization of the sequential type of Nazarov/Michael reaction of pyrrole derivatives. Furthermore, we demonstrated the recyclable use of the iron catalyst and obtained the desired Nazarov/Michael reaction products in good yields for five repetitions of the reactions without any addition of the catalyst using an ionic liquid, [bmim][NTf2 ], as the solvent. We expect that the iron-catalyzed Nazarov cyclization, in particular, in an ionic liquid solvent might become a useful method to synthesize functional molecules that include cycloalkene moieties.

Catalytic Intramolecular Aminofluorination, Oxyfluorination, and Carbofluorination with a Stable and Versatile Hypervalent Fluoroiodine Reagent

Application of a fluoroiodine analogue of the Togni reagent was studied in fluorocyclization reactions. In the presence of a transition-metal catalyst the applied fluoroiodine reagent can be used for aminofluorination, oxyfluorination, and carbofluorination reactions. The described procedure has a very broad synthetic scope for preparation of functionalized hetero- and isocyclic compounds having a tertiary fluorine substituent.

Vapor-phase metalation by atomic layer deposition in a metal-organic framework

Metal-organic frameworks (MOFs) have received attention for a myriad of potential applications including catalysis, gas storage, and gas separation. Coordinatively unsaturated metal ions often enable key functional behavior of these materials. Most commonly, MOFs have been metalated from the condensed phase (i.e., from solution). Here we introduce a new synthetic strategy capable of metallating MOFs from the gas phase: atomic layer deposition (ALD). Key to enabling metalation by ALD In MOFs (AIM) was the synthesis of NU-1000, a new, thermally stable, Zr-based MOF with spatially oriented -OH groups and large 1D mesopores and apertures.

Catalytic asymmetric alkylation reactions for the construction of protected ethylene-amino and propylene-amino motifs attached to quaternary stereocentres

An efficient catalytic and stereoselective method for the direct construction of protected ethylene-amino and propylene-amino scaffolds attached to quaternary stereocentres is reported. Preliminary investigations revealed a mild base catalysed nucleophilic ring opening of N-sulfonyl aziridines using the commercially available phosphazene base 2-tert-butylimino-2-diethylamino-1,3-dimethyl-perhydro-1,3,2-diazaphosphorine (BEMP) was possible and resulted in highly efficient alkylation reactions with a range of methine carbon acids. This reaction could be rendered highly asymmetric (up to 97% ee) by employing phase-transfer catalysis to control stereoinduction. Incorporation of alkyl substituents onto the aziridine electrophile, resulted in a highly diastereoselective (up to 30:1 d.r.) variant of this methodology. A further extension using N-protected cyclic sulfamidates as the electrophilic component was successful with a range of pro-nucleophiles (up to 96% ee and 45:1 d.r.) and allowed a range of nitrogen protecting groups (carbamate, sulfonyl, phosphonyl, benzyl) to be incorporated into the alkylation adducts. Finally, the utility of the products have been demonstrated in the synthesis of useful heterocycles and compounds bearing structural components of natural products.

Organocatalytic asymmetric one-pot sequential conjugate addition/dearomative fluorination: synthesis of chiral fluorinated isoxazol-5(4H)-ones

A facile one-pot sequential conjugate addition/dearomative fluorination transformation of isoxazol-5(4H)-ones with nitroolefins and N-fluorobenzenesulfonimide (NFSI) has been developed. By using a bifunctional chiral tertiary amino-thiourea catalyst, a series of chiral fluorinated isoxazol-5(4H)-ones containing one fluorine-substituted quarternary stereocenter were obtained in high yields with high enantio- and diastereoselectivities. Further transformation of adducts could afford isoxazolidin-5-one derivatives with three contiguous stereocenters.

Developments in fluorocyclization methodologies

Fluorinated organic compounds constitute a significant proportion of medicines marketed today. Since heterocycles are submotifs frequently encountered in lead compounds, the corresponding fluorinated molecules that possess coupling functional groups to increase structural complexity are sought-after building blocks, especially those with stereogenic elements. To access fluoro-heterocycles, fluorocyclizations constitute an important category of reactions that permit multiple bond construction in one pot. Reactions featuring both nucleophilic and electrophilic sources of fluorine have proved valuable for the delivery of fluorinated carbo- and heterocycles. Mechanistically, two scenarios have been validated with the fluorination occurring either prior to or after the cyclization event. Fluorinated biologically active molecules prepared by employing a fluorocyclization protocol are rare, with two notable exceptions being the synthesis of fluorogypsetin and fluorobrevianamide E. Various levels of diastereocontrol were obtained with best results observed when the cyclization step precedes the fluorination. To date, asymmetric fluorocyclizations have not been explored, with the exception of a Nazarov fluorination process. In essence, this process features a catalytic asymmetric cyclization followed by a diastereoselective fluorination. Asymmetric fluoroheterocyclizations are, however, not known. For this methodology to serve medicinal chemistry, conceptual advances are essential to access fluorinated pharmacophores with programmable stereocontrol as and when necessary.

Facile stereoselective synthesis of fluorinated flavanone derivatives via a one-pot tandem reaction

A series of fluorinated flavanones were synthesized in moderate to good yields with excellent diastereoselectivities under mild reaction conditions via a one-pot tandem procedure involving a proline-catalyzed Knoevenagel condensation, a Michael addition, and an electrophilic fluorination by NFSI.

Catalytic Stereoselective Synthesis of Highly Substituted Indanones via Tandem Nazarov Cyclization and Electrophilic Fluorination Trapping

A new catalytic stereoselective tandem transformation via Nazarov cyclization/electrophilic fluorination has been accomplished. This sequence is efficiently catalyzed by a Cu(II) complex to afford fluorine-containing 1-indanone derivatives with two new stereocenters with high diastereoselectivity (trans/cis up to 49/1). Three examples of catalytic enantioselective tandem transformation are presented.